[1] Field measurements and incubation techniques were used to determine the dynamics of acetate formation, iron reduction, and methanogenesis in surficial peat of an Alaskan bog. Acetate concentrations were $100 mM early in the season and decreased to $20 mM in July when the water table decreased. Acetate levels increased rapidly to $1000 mM when the water table rose to the surface in August. Acetate production in anaerobic slurries occurred at rates of 2.8-420 nmol carbon mL À1 day À1 , which was 7-120 times more rapid than CH 4 production. Experiments utilizing 14 C-acetate confirmed that methanogenesis was not acetoclastic although acetate was converted very slowly to CO 2 . Peat incubated anaerobically for 4.5 months at 24°C never produced methane from acetate, suggesting that anaerobic acetate accumulation would have occurred all season if the water table had remained high. CO 2 production was the most rapid process measured in laboratory incubations (up to 750 nmol mL À1 day À1 ) and appeared to be due primarily to fermentation. Acetate was the primary organic terminal product of anaerobic decomposition in the bog, and acetate was ultimately oxidized to CO 2 via aerobic respiration and to a much lesser extent anaerobically by Fe reduction.

[1] Laboratory incubations, gas and solute analyses, and stable isotope methods were used to investigate the pathway of methanogenesis in 25 wetland peats of varying vegetation composition along a latitudinal gradient in Alaska. Sites were divided into gross vegetation classes indicative of tropic status: mostly Sphagnum (class 1); Sphagnum plus vascular plants (i.e., Carex) (class 2); mostly vascular plants, but still containing Sphagnum (class 3), and; sites dominated by vascular plants with no visible Sphagnum species (class 4). The magnitude of CO 2 , acetate and CH 4 as end products of anaerobic metabolism varied greatly, but ratios of end product formation indicative of differences in the pathway of C flow and methanogenesis corresponded with vegetation classes, especially at the extremes, e.g., acetate-C accounted for 67% of total C production in Sphagnum-rich sites (class 1) decreasing to 13% in sites devoid of sphagna (class 4). Conversely, CH 4 comprised only 0.4% of products in class 1 sites, but increased to 14% in class 4. Total respiration rates (sum of all three products) varied by only a factor of $2 among vegetation classes (200-440 nmol ml À1 day À1 ), but rates differed greatly if acetate formation was not included suggesting that belowground C cycling can be much more rapid than previously thought. Apparent fractionation factors (a = d 13 C DIC + 1000/d 13 C CH4 + 1000) that estimate methanogenic pathway, i.e., the relative contribution of CO 2 reduction or acetate as precursors of methane, varied from $1.030 to $1.080 and agreed with incubation end product ratios, underscoring the importance of the presence or absence of vascular plants and Sphagnum mosses in affecting the pathway of anaerobic C flow. We contend that methanogenesis in general, including CO 2 reduction, is impeded in northern wetlands compared to the production of other C compounds and that its importance decreases with oligotrophy. The connection with vegetation suggests that climate change scenarios leading to increases in vascular plant cover in northern wetlands may shift methanogenic pathways toward increased acetotrophy and increased methane formation, which is a positive feedback on warming.

Although northern peatlands contribute significantly to natural methane emissions, recent studies of the importance and type of methanogenesis in these systems have provided conflicting results. Mechanisms controlling methanogenesis in northern peatlands remain poorly understood, despite the importance of methane as a greenhouse gas. We used 16S rRNA gene retrieval and denaturing gradient gel electrophoresis (DGGE) to analyse archaeal communities in 15 high-latitude peatland sites in Alaska and three mid-latitude peatland sites in Massachusetts. Archaeal community composition was analysed in the context of environmental, vegetation and biogeochemical factors characterized in a parallel study. Phylogenetic analysis revealed that Alaskan sites were dominated by a cluster of uncultivated crenarchaeotes and members of the families Methanomicrobiaceae and Methanobacteriaceae, which are not acetoclastic. Members of the acetoclastic family Methanosarcinaceae were not detected, whereas those of the family Methanosaetaceae were either not detected or were minor. These results are consistent with biogeochemical evidence that acetoclastic methanogenesis is not a predominant terminal decomposition pathway in most of the sites analysed. Ordination analyses indicated a link between vegetation type and archaeal community composition, suggesting that plants (and/or the environmental conditions that control their distribution) influence both archaeal community activity and dynamics.

We examined the seasonal changes of the cecal microbiota of captive arctic ground squirrels (Urocitellus parryii) by measuring microbial diversity and composition, total bacterial density and viability, and short-chain fatty acid concentrations at four sample periods (summer, torpor, interbout arousal, and posthibernation). Abundance of Firmicutes was lower, whereas abundances of Bacteroidetes, Verrucomicrobia, and Proteobacteria were higher during torpor and interbout arousal than in summer. Bacterial densities and percentages of live bacteria were significantly higher in summer than during torpor and interbout arousal. Likewise, total short-chain fatty acid concentrations were significantly greater during summer than during torpor and interbout arousal. Concentrations of individual short-chain fatty acids varied across sample periods, with butyrate concentrations higher and acetate concentrations lower during summer than at all other sample periods. Characteristics of the gut community posthibernation were more similar to those during torpor and interbout arousal than to those during summer. However, higher abundances of the genera Bacteroides and Akkermansia occurred during posthibernation than during interbout arousal and torpor. Collectively, our results clearly demonstrate that seasonal changes in physiology associated with hibernation and activity affect the gut microbial community in the arctic ground squirrel. Importantly, similarities between the gut microbiota of arctic ground squirrels and thirteen-lined ground squirrels suggest the potential for a core microbiota during hibernation.A lthough gut microbes share a mutualistic relationship with their mammalian hosts in which they benefit from access to fermentable substrates and a suitable living environment (1), the gut microbiota may be exposed to periods of little or no available dietary polysaccharides when the host is fasting. Extended periods of host fasting likely select for microbes that are able to degrade and subsist on host-derived substrates such as mucins and other glycoproteins (2-4). Indeed, several studies have revealed a profound effect of fasting on the gut microbiota. For example, fasting Burmese pythons (Python molurus) have higher relative abundances of Bacteroidetes, a phylum with species able to utilize hostderived substrates (1), whereas greater relative abundances of Firmicutes, which rely upon diet-derived substrates (5), were observed after the ingestion of a meal (6). Similarly, fasted Syrian hamsters (Mesocricetus auratus) exhibit decreased bacterial densities, relative abundance of Firmicutes, and microbial metabolic activity compared to those of fed hamsters (7).Many obligate seasonal hibernators (e.g., ground squirrels) naturally exhibit an endogenous circannual rhythm of hibernation and activity (reviewed in reference 8). During hibernation, animals voluntarily fast, and they conserve endogenous energy reserves by entering a state of torpor characterized by days to weeks of profoundly reduced metabolic rate, body temper...

Abstract. High latitude wetlands are significant sources of atmospheric methane, with emission rates that are susceptible to effects of climate change. Our data demonstrate that unlike mid-latitude wetlands, methane in northern peatlands is not derived from acetate or C 1 compounds. These latter compounds accumulate to high levels with acetate as the primary organic end product of anaerobic decomposition. Acetate is ultimately degraded aerobically to carbon dioxide after diffusion into oxic regions of peat. Therefore, organic precursors destined for methane in mid-latitude wetlands are degraded to carbon dioxide in northern wetlands. A warming-induced initiation of acetoclastic methanogenesis could substantially increase methane production.